This chapter will examine characteristics of three types of electrical interconnection mediums that fall under the umbrella of cable design, which are as follows: 1. HV cable design This comprises the main interconnecting AC cables for power transmission and distribution, broadly covering the network voltages of 33 up to 400 kV - although 11 kV cables will be discussed where relevant. 2. Gas insulated transmission lines (GILs) Gas insulated transmission line (GIL) is included as part of this chapter since when installed, it generally serves as a replacement for HV cables. 3. Auxiliary cables Auxiliary cables refer to LV (i.e. low voltage) cables for interconnecting protection, control and telecommunications equipment and will include the following: (i) Multi-core cables (ii) Multi-pair cables (iii) Fibre optic cables.